A difficult obstacle associated with the exploration and production of oil and gas is management of significant ocean currents. These currents can produce vortex induced vibration (VIV) and/or large deflections of tubulars associated with drilling and production. VIV can cause substantial fatigue damage to the tubular or cause suspension of drilling due to increased deflections. Both helical strakes and fairings can provide sufficient VIV suppression, but can be slow and unsafe to install.
Most suppression devices are placed against the tubular. For example, helical strakes are typically clamped to the tubular, though in some cases the helical strake may have a spacer element to allow water flow under the strake for cathodic protection. Fairings typically have a small annulus between the fairing and the tubular, but are held in the same axial position by collars that are clamped to the tubular.
Fairings are typically free to weathervane (rotate) about the longitudinal axis of the tubular, and are supported by collars to keep them from sliding along the tubular axis more than desired. Often, collars are used at each end of the fairing to maintain the desired axial position.
A problem associated with collars is that it is difficult to design a collar that is able to accommodate other exterior or auxiliary tubulars outside of the main, or central, tubular. Since the collars are typically clamped tight against the main tubular, and since adjacent tubulars are often not designed to take the large forces associated with the tight collar, the presence of adjacent exterior tubulars is difficult to accommodate. This is especially true of drilling risers, which may have several lines running outside of the main pipe, for example choke and kill lines, booster lines, and other lines.
Sometimes fairings are installed on tubulars with buoyancy, insulation, or other coatings that contain the adjacent tubulars and allow the collar to be installed on a strong structure. Many fairing systems have been produced using collars that clamp around the buoyancy or insulation. However, and in particular for drilling systems, it is desirable to be able to use the same fairing system for both joints having buoyancy and joints for which the adjacent tubulars are exposed.
One solution is to build a very large collar and cut slots for adjacent tubulars positioned along the outer surface of the main tubular. Unfortunately, this is not very practical with the main tubular, which is large, and/or the distance between the main tubular and the adjacent tubulars is substantial. In addition, one important consideration is for the collars to be fast and/or inexpensive to install. Making the collars larger, however, is not very cost effective.